For the reasons outlined by Guido and Iain Dale, it was probably a slight mis-step by the coalition – but not one, in itself, that will have any important rammifications for them or the public. For while it's not the government's prerogative to decide on the entire Question Time panel, it is their prerogative to choose whether or not they join it. Last night, they simply chose not to.
This does inspire a wider question, though, about how the government will deal with the media in future. There's a sense in coalition circles that many media appearances will just reduce into attempts to conjure a split story between the Tories and the Lib Dems – especially when an agent provocateur like Campbell is involved. So don't be surprised to see Downing St being fussy about Question Time, and other shows, in future.